Like fruits and vegetables, oils pack the most nutrients when they're unheated and unrefined. "The heating process by which manufacturers develop commercial oils is to about 450 degrees, and the extraction process changes the nature of the oil molecule," says Sheehan McGuirk, aesthetician for Priti Organic Spa in New York. Cold-pressed oils that haven't been extracted with solvents retain more vitamins and minerals, but because the method is more time-consuming, they cost more. McGuirk also recommends glass containers, and suggests being diligent about keeping plant oils in cool, dark places. Oils can go rancid in six months, so follow your nose to determine freshness—fresh oils should smell like the plants they're derived from.